Magneto-rheological damper

ABSTRACT

What is disclosed is a magneto-rheological (MR) damper including a piston guided in a cylinder that is filled with a magneto-rheological fluid, its work volume being formed oblique relative to the piston axis between the piston and the cylinder.

[0001] The invention concerns a magneto-rheological (MR) damper inaccordance with claim 1.

[0002] MR dampers include a magneto-rheological fluid, a work volume towhich the fluid is applied, and an element for generating a magneticfield, wherein the yield limit of the fluid may be modified byapplication of the magnetic field.

[0003] A housing cavity is divided by a piston into a piston-sweptvolume and a cylinder volume, with the fluid being capable of flowingbetween the volumes through openings. In principle, MR dampers operatingin the Valve Mode, Shear Mode, or Squeeze Mode are known. Frequently thesingle principles are mixed, however, so that an accurate classificationis not possible.

[0004] An MR damper operating, e.g., in the valve and shear modes isdisclosed in patent specification U.S. Pat. No. 5,398,917 FIG. 3. Theopenings are formed between the moving piston and the housing innerwall. The fluid in the gap is subjected to the magnetic field by anelectrical coil radially arranged on the piston. In the event of axialmovements of the piston, the fluid flows through the openings from thecylinder volume into the piston-swept volume or vice versa. Owing to theratio of free cross-section/cylinder (piston-swept) volume <1, themovement of the piston is damped correspondingly. The opening acts as avalve, the effect of which may be increased or reduced by a change inviscosity.

[0005] Moreover the fluid flowing through the openings is sheared acrossthe free cross-section on the stationary housing inner wall and themoving piston, resulting in additional damping.

[0006] It is a drawback in this solution that the coil is mounted on thepiston, so that an adaptation of the MR damper to the conditions of therespective task (damping properties) by simple and rapid replacement ofthe coil is not possible. Optimum adaptation of the MR damper todifferent damping conditions accordingly is not possible.

[0007] It is another drawback of the known solution that damping takesplace not so much through the viscosity of the fluid than rather thecross-section of the openings in the piston and the related throttlingeffect. I.e., the flow and creep properties of the magneto-rheologicalfluid are not put to optimum use.

[0008] It is another drawback of this solution that when a load isapplied, the valve mode may possibly create a considerable pressuredifferential between the cylinder-swept volume and the cylinder volume,with all the components of the MR damper, in particular seals, having tobe designed with corresponding adaptations.

[0009] An MR damper in which the coil is arranged externally of thehousing cavity and which does not operate in the valve mode, isdisclosed in U.S. Pat. No. 5,492,312. The MR damper operates in thesqueeze mode, so that by application of a magnetic field by means of thecoil, which extends through the full length of the MR damper, the entirehousing cavity and thus the entire fluid is subjected to the magneticfield. Here the fluid may flow through an annular space between thepiston and the housing inner wall between the cylinder volume and thepiston-swept volume.

[0010] It is a drawback of this solution that the entire housing cavityis exposed to the magnetic field, with damping accordingly beingachieved not so much by squeezing the fluid in the piston-swept volumeor through the annular space, but rather by pinching of the piston inthe fluid.

[0011] It is an object of the present invention to furnish an MR damperwhich eliminates the named drawbacks and thus exhibits substantiallyimproved response characteristics.

[0012] This object is attained through the features of claim 1.

[0013] In order to attain this object, an MR damper is furnished whichpreferably operates in the squeeze mode and in the shear mode. The valvemode is distinctly reduced. The single axial openings between a cylinderand a piston are in accordance with the invention replaced by an annularspace formed obliquely relative to the piston axis. Here a displacementvolume in continuation of the annular space and a piston cavityconnected with the annular space through bores constitute the cylindervolume (piston cavity) and the piston-swept volume (displacementvolume). The valve effect is reduced through a correspondingly selecteddiameter of the bores relative to the inner width of the annular space.For generating a magnetic field in the annular space, an electrical coilis affixed radially outside the cylinder.

[0014] The essential advantage of the preferred arrangement of theelectrical coils externally of the housing cavity resides in the factthat the electrical coils may be replaced rapidly and easily, so that acorresponding adaptation of the MR damper to the conditions of use maybe realized.

[0015] Once the piston is moved in an axial direction, the fluid opposesto the piston force a squeeze force and a shear force, whereby thepiston is damped accordingly. The squeeze and shear forces i.a. dependon the viscosity, the diameter of the bores, and the oblique orientationof the annular space relative to the piston axis.

[0016] A preferred embodiment provides to form the MR dampersymmetrically relative to a transverse axis, so that damping may takeplace both in the pulling and pushing directions. The piston cavitycommunicates through additional bores with a second annular space whichopens into a second displacement volume. The second annular space isalso pervaded by a magnetic field from an electrical coil, with thecoils optionally generating opposite-sense or same-sense magneticfields.

[0017] In this case the piston cavity merely is to be considered athrough passage for the fluid into the second displacement volume. Inaccordance with the axial direction of movement of the piston, eitherthe first displacement volume or the second displacement volume thusoperates as a cylinder-swept volume or a cylinder volume.

[0018] In another preferred embodiment, the MR damper is provided withfilling and bleeding bores. Moreover the bores between the annularspaces and the piston cavity are positioned closer to the piston axis,whereby the squeeze mode is emphasized more strongly.

[0019] A particular field of application of the MR damper in accordancewith the invention is damping of high frequencies as occur, e.g., inrailway facilitites. When correspondingly installed in the track beddingor in the foundation, it is in particular possible to suppress highfrequencies.

[0020] Further advantageous embodiments are subject matters of thesubclaims.

[0021] Preferred embodiments are explained in more detail in thefollowing schematic representations, wherein:

[0022]FIG. 1 is a sectional view of a magneto-rheological damper inaccordance with the invention, and

[0023]FIG. 2 is a sectional view of a magneto-rheological damper inaccordance with the invention having filling and bleeding bores.

[0024]FIG. 1 shows a magneto-rheological (MR) damper 2 in accordancewith the invention which includes a piston 4 on a magneticallynon-conductive piston rod 5, which is guided in a damper cavity 6 of acylinder 8 filled with a magneto-rheological fluid. The MR damper 2 isof a symmetrical design relative to a transverse axis 12 extendingtransversely to the longitudinal axis 10, whereby a damping stroke inboth axial directions of movement is achieved, so that tensile andcompressive forces may be damped.

[0025] The piston 4, also referred to as an armature, forms between itsinner cone surfaces 20, 22 of the cylinder 8 and its correspondinglyshaped outer cone surfaces 24, 26 two annular spaces 16, 18 extendingobliquely to the piston axis 10. Owing to the oblique orientation of theannular spaces 16, 18 the annular space volume is increased in contrastwith axially oriented annular spaces, so that correspondingly more fluidis available for damping of the piston 4. The inner cone surfaces 20, 22of the cylinder 8 are formed by two piston counterparts 28, 30 inaxially opposite positions, also referred to as armature counterparts.The piston counterparts 28, 30 are inserted in a pole tube 32 and spacedapart so that in a range between them on the inner peripheral surfaces34 of the cylinder 8 a guide surface 36 for the piston 4 in the cylinder8 is formed. Between guide surface 36 and piston 4 there is a fluidfilm. This latter is strained in the shear mode and thus assumes thetask of the shear mode, i.e., upon application of a magnetic field theshearing strain (viscosity) of the medium (MR fluid) is increased. As aresult of the increased shearing strain, movement of the piston(armature) is opposed by an increased kinetic resistance. It is to benoted that—although the fluid film is formed by the magneto-rheologicalfluid, fluid does not flow along the guide surface 36 between theannular spaces 16, 18.

[0026] The annular spaces 16, 18 extend between the inner cone surfaces20, 22 and the outer cone surfaces 24, 26 radially between internaldisplacement volumes 38, 40 and the external inner peripheral surface 34of the pole tube 32. In the center portion of the annular spaces 16, 18,bores 42 are formed in the piston 4 which are in communication with apiston cavity 43, so that the fluid may flow between the displacementvolumes 38, 40 in accordance with the axial movement of the piston 4.Flow of the fluid through the piston cavity 43 allows to guide thepiston 4 in accordance with MR dampers in the valve mode, wherein otherthan with known MR dampers in accordance with the squeeze and shearmodes, the slide surface is not provided with through passages. Thedisplacement volumes 16, 18 are separated from the environment by seals44, 46, e.g. membrane seals radially arranged around the piston axis inthe piston counterparts 28, 30,.

[0027] Radially outside of the damper cavity 6 two electrical coils 48,50 are arranged side by side around the cylinder 8. The electrical coils48, 50 encompass the pole tube 32 and are each sheathed by magneticallyconductive segments 52, 54, 56. Axial securing of the electrical coils48, 50 is achieved through spacer rings 58, 60 between flange-type endportions 62, 64 of the piston counterparts 28, 30. The electrical coils48, 50 may be driven independently and in particular extend through theannular spaces 38, 40 with one magnetic field each, with the magneticfields preferably being opposite to each other. This has the advantagethat the piston 4 in the cylinder 8 automatically returns into a startposition after damping when both coils 48, 50 are driven, so that anexternal device for resetting the piston, such as a spring, is notnecessary. It is an additional advantage of this arrangement that the MRdamper of the invention may be utilized as a differential throttle pathmeasurement device, so that the damping stroke can be measured inparallel with damping.

[0028] In order to avoid superpositions of the two magnetic fields, twomagnetically non-conductive rings 66, 68 divide the pole tube 32 intothree magnetically conductive segments 70, 72, 74. The number ofmagnetically non-conductive rings 66, 68 depends on the number ofelectrical coils 48, 54.

[0029] In the event of axial movements of the piston 4, the fluid bothin the displacement volume 38, 40 and in the annular space 16, 18 issqueezed, with shear taking place in the opening range of the bores 42.Damping essentially takes place as a result of squeeze in the annularspace 38, 40 above the bores 42, for the magnetic field is primarilyapplied to annular space 38, 40, and the fluid may flow from thedisplacement volume 16, 18 via the annular space 38, 40 into the bores42 and thus into the piston cavity 43 or the opposite displacementvolume 16, 18, respectively.

[0030] In order to support the shear effect, the diameters of the bores42 are selected such as to be greater than the inner width of theannular spaces 38, 40. Accordingly the bores 42 practically do notconstitute a throttling location for the fluid flow, so that adifferential pressure does not build up between the openings of thebores 42.

[0031] The effect of the valve mode accordingly is of secondaryimportance. Although it may be intensified by reducing the diameters ofthe bores 42 relative to the inner width of the annular spaces 38, 40,this measure nevertheless results in a reduction of the shear andsqueeze effects.

[0032] A damper stroke may in principle be determined through the designof the MR damper and the viscosity of the fluid (yield limit inproportion to the applied magnetic field). It is, for example, possibleto modify the structure of the MR damper to the effect of changing theratio diameter of bores 42/inner width of the annular spaces 16, 18 inorder to intensify the valve mode, or to choose a modified obliqueorientation of the annular spaces relative to the piston axis in orderto further emphasize the shear and squeeze modes. The yield limit may beraised, for example, by extending the magnetic field to the displacementvolume 38, 40.

[0033]FIG. 2 shows an MR damper with filling and bleeding bores 76. Thefilling and bleeding bores 76 extend on the one hand in the pistoncounterparts 28, 30 approximately axially between the displacementvolumes 38, 40 and axially extending recesses 78 in the flange-type endportions 62, 64, and on the other hand radially through the piston 4from the piston cavity 43 in a direction towards the guide surface 36.The recesses 78 are designed as filling openings and may be opened orclosed with the aid of a corresponding closure, preferably screw plug.

[0034] Due to the arrangement of the filling and bleeding bores 76,trapping of air cannot occur when the filling openings are openedsimultaneously during filling, and residual amounts of old fluid cannotgather in the MR damper 2 when the fluid is exchanged.

[0035] In addition, the bores 42 in the piston 4 are arranged closer tothe piston rod 5. This has the advantage that the range 80 of theannular spaces 16, 18 located above the bores 42 is prolonged, wherebythe squeeze effect is intensified.

[0036] One embodiment provides for the electrical coils 48, 50 to bearranged not externally of, but inside the damper cavity 6, e.g.radially on the piston 4.

[0037] Another embodiment provides to generate required magnetic fieldsnot through electrical coils 48, 50 but through other suitable elementssuch as, for instance, magnets. The number of elements here depends onthe damper force to be applied. It is conceivable for not only a annularspace 18, but other ranges of the damper cavitys 6 or the entire dampercavity 6 being subjected to magnetic fields.

[0038] Another embodiment provides for the fluid not to flow through apiston cavity 43 between a cylinder-swept volume and cylinder volume,but an internal space being formed between the piston 4 and an innerperipheral surface 34.

[0039] What is disclosed is a magneto-rheological (MR) damper includinga piston guided in a cylinder that is filled with a magneto-rheologicalfluid, its work volume being formed oblique relative to the piston axisbetween the piston and the cylinder.

1. A magneto-rheological (MR) damper including a piston guided in adamper cavity of a cylinder that is filled with a magneto-rheologicalfluid, the rheological properties of which may be modified byapplication of a magnetic field; and wherein concurrently with amovement of the piston the fluid is pushed from a work volume into areception space, and wherein the work volume is formed by an annularspace extending obliquely relative to the piston axis between the pistonand cylinder.
 2. A MR damper in accordance with claim 1, wherein thepiston through its outer peripheral surface defines the annular spaceand through its piston rod forms a piston cavity which communicates withthe annular space through at least one bore.
 3. A MR damper inaccordance with claim 1, wherein the bore is arranged in the centerrange of the annular space in the piston, the bore diameter beingselected such that the inner width of the annular space is substantiallysmaller.
 4. A MR damper in accordance with claim 1, wherein the annularspace is formed by inner cone surfaces of the cylinder andcorrespondingly formed outer cone surfaces of the piston.
 5. A MR damperin accordance with claim 4, wherein the inner cone surface are formed bya piston counterpart inserted into a pole tube, the inner peripheralsurface of which defines the annular space in the range between thepiston counterparts placed at a distance from each other.
 6. A MR damperin accordance with claim 1, wherein in continuation of the annular spacea displacement volume is formed.
 7. A MR damper in accordance with claim1, wherein the MR damper is designed to be symmetrical relative to atransverse axis extending transversely to the longitudinal axis
 8. A MRdamper in accordance with claim 2, wherein the magneto-rheological fluidflows through the piston cavity between displacement volumes.
 9. A MRdamper in accordance with claim 1, wherein the damper cavity is axiallyseparated from the environment by seals, preferably membrane seals. 10.A MR damper in accordance with claim 5, wherein the pole tube ispreferably divided by two magnetically non-conductive rings into threemagnetically conductive segments.
 11. MR damper in accordance with claim1, further comprising elements, for generating a magnetic field in theannular spaces.
 12. A MR damper in accordance with claim 11, wherein theelements are two electrical coils for generating magnetic fields, thecoils being applied externally of the annular spaces.
 13. A MR damper inaccordance with claim 1, wherein the MR damper includes filling andbleeding bores, with some filling and bleeding bores opening intofilling openings preferably adapted to be opened and closed with the aidof a screw plug.
 14. A MR damper in accordance with claim 13, whereinthe filling and bleeding bores extend approximately axially through thepiston counterparts from the filling openings to the displacementvolumes, and radially through the piston from the piston cavity in thedirection of the guide surface (36)
 15. A MR damper in accordance withclaim 13, wherein the filling openings are opened and closed using ascrew plug.